Stepmotors now close the loop

Stepmotor drive systems are known for their inherent open-loop control simplicity. But compared to servo systems, they perform poorly in applications requiring high speed, high acceleration, and smooth motion. Moreover, steppers excite mechanical resonance, stall on occasion, and are noisy. One company, however, says these problems are resolved.

Agile Systems, Waterloo, Ont., Canada, developed a new stepmotor control approach, called Silentstep, that lets steppers run like servos while maintaining their inherent simplicity. With Silentstep, stepmotors operate continuously, rather than in incremental steps, deriving position information from motor back-EMF. The motors start up in conventional microstepping mode, then switch over (to Silentstep) at the back-EMF threshold speed. The switchover typically takes place at frequencies well below levels where audible noise and resonance effects are a problem.

Silentstep controllers have either twin full-bridge drive circuits or three half bridges and work with bipolar stepmotors. Using three half bridges reduces the number of transistors from eight to six, allowing the technology to be deployed on conventional bldc motor drivers. The only drawback is that it reduces phase voltage (by half), as well as usable motor speeds.

Silentstep compensates for the phase voltage reduction by using flux-vector control, which gives the ability to implement magnetic field optimization and achieve motor speeds beyond the typical bus voltage limitation. To work, the controller derives motor current commands from the setpoint and actual position and speed. When the magnitude of the voltage vector is maximized, the controller automatically phase adjusts the vector so the motor runs beyond its normal open-loop mode stalling point.